Interpretive Summary: Lake Champlain continues to be polluted by excess phosphorus (P)from farm runoff despite many control efforts because the efforts are voluntary and not guided by a watershed-wide plan. A dairy-based, agricultural subwatershed was modeled with SWAT and GIS to locate critical source areas (CSAs), which identify where control measures would be most helpful. Critical source areas were located for controlling phosphorous losses through both runoff and erosion. The identification of CSAs for P loss is expected to support the next phase of our project, which involves exploring cost-effective P management strategies with the highest potential for P loss reduction applicable to the study watershed and Lake Champlain Basin.

Technical Abstract:
Lake Champlain, located between Vermont, New York, and Quebec, exhibits eutrophication due to continuing phosphorus (P) inputs mainly from upstream nonpoint source areas. To address the Lake's eutrophication problem and as part of total maximum daily load (TMDL) requirements, a state-level P reduction goal has been set by the Department of Environmental Conservation of both Vermont and New York. Unfortunately, remedial measures undertaken thus far to control the nonpoint P losses have been primarily based on landowner voluntary participation and have not been guided by a systematic technique to implement remedial measures where they are most needed (greater P loss risk) and where they can provide the greatest P loss reduction. Consequently, P reduction goals have not been achieved in most segments of Lake Champlain. The main objective of this study was to identify land uses with the highest P loss - i.e., critical sources areas (CSA) - using a model-based approach. Soil and Water Assessment Tool (SWAT) is used for this objective. This study focuses on the Rock River Watershed, which is one of the highest contributors of P to Lake Champlain. Spread over 71 km**2, the watershed is dominated by dairy agriculture and has fertile periglacial lacustrine and alluvial soils with an old tile drainage system. SWAT predictions of hydrology, sediment, and P loss are being evaluated for Rock River Watershed. In this paper, outputs of model calibration, validation, and spatial locations of CSAs of runoff, sediment, and P losses are presented. The identification of CSAs for P loss is expected to support the next phase of our project, which involves exploring cost-effective P management strategies with the highest potential for P loss reduction applicable to the study watershed and Lake Champlain Basin.